1. Field of the Invention
The present invention relates to a distributed communications network, that is to say a communications network in which the control of data transmission is not localised but is distributed between a plurality of nodes in the network.
2. Description of Related Art
Hitherto, most networks have been arranged to provide either a connection-oriented service or a connectionless service. In a connection-oriented service, a circuit is established between a source node and a destination node of the network, and all the data transferred between the source node and destination node travels via the links and nodes incorporated in the circuit. The circuit is released only when the call is terminated. Such an arrangement has the advantage that all the data arrives at the destination node in the same order as it is transmitted from the source node. In a connectionless service, data is transmitted between a source node and a destination node via packets (generally referred to as datagrams) which each include the address of the destination node and travel along one or more paths through the network to the destination node, the paths chosen generally depending upon the conditions in the network. Accordingly the datagrams for the source node can reach the destination node via a multiplicity of paths through the network, each having different delays so that the order in which the datagrams are received at the destination node does not necessarily correspond to the order in which the datagrams are transmitted from the source node. Accordingly, facilities are normally required to re-order the data received by a destination node of such a network.
For a general discussion of computer networks, reference is made to Tanenbaum "Computer Networks" 2nd Edn. (1989) Prentice-Hall Inc., particularly the OSI reference model shown in FIGS. 1-7 (p. 15). In this model the network is conceptually divided into seven layers. The following description relates mainly to layer 3, the network layer.
Some of the more important networks are summarised below: TYMNET is a commercial value-added network operated by ST Tymnet Inc of California. It was set up in 1971 and currently has approximately 1000 nodes in the USA and Europe. Routes in TYMNET are computed by a central Supervisor node, which has its own representation of the complete network. A route computation is carried out every time a circuit is needed. A path is set up using a NEEDLE packet, which knows its route when it is generated by the Supervisor. This packet travels to the source of the call and then travels to the destination node, setting up a circuit as it goes. The circuit is dismantled using a ZAPPER packet.
Packets are forwarded over the constructed circuit by using logical channel associations set up by the NEEDLE packet. In TYMNET, link costs are assigned by the Supervisor according to the link capacity, transmission facilities, the type of sessions, and so on. To help with this, each node periodically sends status reports to the Supervisor. In TYMNET the link cost function is based partially on the link load; when the load changes sufficiently the Supervisor is informed. On failure all nodes incident to the failure inform the Supervisor, all circuits using failed links are dismantled, and the Supervisor then generates fresh circuits and creates them using NEEDLEs.
Thus TYMNET is a centralised communications network which is connection-oriented. In particular, it should be noted that both the NEEDLE and the ZAPPER packets travel in predetermined circuits and are therefore not classified as datagrams.
ARPANET (Advanced Research Projects Agency Network) was the first network in the USA to use packet-switching technology. It was set up as an experimental network to investigate system resource sharing in 1969, and currently connects approximately 60 nodes in the USA and Europe. Routes are generated in ARPANET in a distributed fashion. Each node maintains a network database, which includes a complete network topology and the link costs. Each node broadcasts the link costs of its incident links to all other nodes periodically. ARPANET uses datagrams, so that circuits do not have to be constructed. Each node calculates a Routeing Table (RT) from its delay table and topology database; this RT contains the best outgoing link to all nodes in the network. Packet routeing in ARPANET is datagram-oriented. An end-to-end protocol is used to ensure that packets are received at the destination in the correct order.
The link cost is the average packet delay suffered by a packet over the link during a 10-second interval. Thus packet delay includes queueing and processing. In ARPANET the link cost function is based partially on the link load, when the load changes sufficiently all other nodes in the network are informed. Also, since the routeing decision is made at the nodes without concern for the session, packets tend to be steered towards areas of low congestion. Network component failure triggers a link cost update and hence a route generation update; this is essentially no different from other link cost changes apart from the topology changing. When a link fails, all packets queued for that link are discarded.
Thus ARPANET is a connectionless distributed data communications network.
DECnet is any network which uses the Digital Network Architecture developed by the Digital Equipment Corporation. In DECnet routes are computed in a distributed fashion. Whenever a link cost changes, the information ripples out through the network. The link costs are based on the full capacity of the link, and are assigned by the system manager. They are updated only when the transmission facilities change status, e.g. failure. Routeing in DECnet is datagram-oriented, so the transport layer orders the packets at the destination. Network component failure triggers a link cost update and hence a route generation update; this is essentially no different from other link costs changes apart from the topology changing. When a link fails, all packets queued for that link are discarded.
Thus the DECnet network is entirely connectionless (i.e. only datagrams are used) and in most respects is distributed, although the link costs are assigned by a central network manger node. MERIT is a small network, intended to connect the computers belonging to the universities and colleges in the state of Michigan, USA. In MERIT routes are generated in a distributed fashion. Whenever a link cost changes, information ripples out through the network. This information is in the form of NETCHANGE messages sent between neighbours; the receiving node then updates its tables and may itself send NETCHANGE messages. The link cost in MERIT is based on hop-count only.
Failure may cause the hop-count in a NETCHANGE message to be increased to the number of nodes in the network, in which case it is assumed that the appropriate node is unreachable. Routeing in MERIT is datagram-oriented.
Thus the MERIT network is a distributed, connectionless network. It will be noted that each of the above networks is either a centralised, purely connection-oriented network or is a distributed, connectionless (i.e. datagram-oriented) network.